Official Luthiers Forum!

Good Morning:
I am considering using French Polish on the top of my latest and KTM9 on the rest of the body. If I understand correctly, when I wet sand the KTM if I get any water on the shellac it will damage it. Do I have to finish the lacquer completely before I do the top? Whats the sequence?
Thanks for being here everyone.......
Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Mikey...I'm about to do the same thing except with Catalyzed Polyester. In order to avoid the same issue that you described, Joe White will apply the Poly first, after which I will apply FP shellac to the top.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Shellac can be wet sanded without a problem if it's dewaxed.

Mikey-
You want to have a nice smooth transition from the FP to the lacquer (somewhere on the binding?), so I'd get the lacquer quite flat and smooth before running the FP over it. That FP layer will be quite thin.

Wet-sanding FP works OK, though I wouldn't leave puddles standing on the surface. Steam to raise dents will mess up FP in a big way.

Cheers
John

Shellac itself is very water resistant. You can usually wet sand a recently applied French polish finish without problems. But as the finish hardens, and the wood moves, microscopic cracks develop, and water can penetrate, and raise the grain. If the grain was not raised with water before finishing, this can be dramatic. Sanding with naptha or mineral spirits prevents this possibility. For cleaning glue squeeze out when gluing the bridge, a damp paper towel works well--just avoid letting water stand on the surface.

Suppose you forget, and start wet sanding a guitar that you French polished a couple of months ago. The wood starts erupting through the finish. (This won't happen unless the customer is coming to pick up the guitar in an hour.) If it does happen, stop. Confess your sins to the customer, get the water off the surface, and wait at least a day for the wood to dry before repairing the finish (just some light sanding with naptha, and a little more French polishing). If you make the repair too soon, you'll sand the tops off all the swollen grain, leaving craters when it shrinks back.

Naturally, nothing like this has ever happened to me.

Just to confirm this in a way:the problem I had with steam and FP was a guitar that had been FP finished a couple of years previously. (I'm not going to experiment with other, more recently FP'ed instruments!)

Eric is mostly correct however Warm water will cause hardend shellac to cloud regardless how fresh the finish is. Tempid water that is allowed to stand for a given amount of time will do the same. if you insist on sanding FP finish then use tempid water and a drop or two of dish soap. personally I never sand FP finishes. I level by spiriting as I buuild the film and I build sheen by glazing process.